The present invention relates to a change-speed transmission and more particularly to an automatic transmission comprising a hydrokinetic unit, such as a torque converter or a hydraulic coupling, a divider in the form of a planetary gear set, and a planetary gear train.
The conventional automatic transmissions using a hydrokinetic unit will provide smooth and effortless start-up of the automobile and shockless shifting as compared to the conventional manual transmissions. However, as there is a slip in a hydrokinetic unit, the automobile equipped with an automatic transmission using a hydrokinetic unit is worse in fuel economy than the automobile equipped with a manual transmission.
In order to improve fuel economy of an automobile equipped with an automatic transmission having a hydrokinetic unit, it has been proposed to provide a torque flow path bypassing the hydrokinetic unit which is selectively established by means of a clutch so that when the first forward speed is to be selected, the clutch is disengaged or uncoupled to make full use of the slip characteristics of the hyrokinetic unit for ease of start-up of the automobile. When another forward speed is to be selected, the clutch is engaged or coupled to establish the torque flow path so as to allow a portion of the input torque to flow through this direct path, although the balance portion of the engine torque is transmitted by the hydrokinetic unit. This torque split arrangement results in substantial reductions in the fuel consumption of the automobile.
There is known a two-speed automatic transmission, embodying the proposal described above, in which all of the input torque on an input member is transmitted through a hydrokinetic unit to provide the first forward speed and in which, to provide the second forward speed, the transmission of a portion of the input torque is effected through a first or torque split clutch bypassing the hydrokinetic unit with the balance portion of the input torque being transmitted by the hydrokinetic unit. This known transmission comprises: an input member; an output member; a hydrokinetic unit having a pump connected to the input member and a turbine; a divider in the form of a planetary gear set which has a ring gear connected to the turbine, a sun gear, and a carrier, which in turn connected to the output member. The sun gear is connectable through a first or forward clutch to the input member. The first clutch is engaged upon selection of the second forward speed to permit a portion of the input torque to be transmitted directly to the output member bypassing the hydrokinetic unit. This automatic transmission has limited application, and particularly is not well suited for an automobile, such as a passenger car of the type in use today, because it has only two forward speeds so that it can not meet the varied demands for every day operating conditions.